Expander for microwave signals



Sept. 15, 1953 c. c. CUTLER I EXPANDER FOR MICROWAVE SIGNALS 2 Sheets-Sheet 1 Filed Sept. 30, 1949 a 2 m/ 6 m m k m m M 2% a! P 9F m m l RELA T/VE VOLTS INPUT INVENTOR g. C. CUTL ER ATmRA/EV Sept. 15, 1953 Filed Sept. 30. 1949 OUTPUT LEI/E db ABOVE I MILL [WATT 2 Sheets-Sheet 2 FIG. 6 1 FIG. 7

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s lo /24 E a E l2 -l4 0 P I2 I 4I I I6 I (a l I PEAK INPUT LEVEL- db naovs MILL IWAT r lNl/E'NTOR g. C. CUTLER Patentecl Sept. 15, 1953 UNITED STATES PATENT OFFICE EXPANDER FOR MICROWAVE SIGNALS Cassius C. Cutler, Gillette, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application September 30, 1949, Serial No. 118,890

.6 Claims. 1

This invention relates to apparatus for modifying the amplitude range of applied waves and more particularly to expanders and compressors for use at microwave frequencies, especially as disclosed. in the application of A. F. Dietrich, Serial No. 118,856 filed on even date.

In microwave systems as in lower frequency systems, it is often desirable to provide transmission characteristics :that are non-linear with the amplitude level of the signal to be transmitted. In frequency modulation systems, for example, it may be desired to provide compression in such a way astto permit ,whatis essentially limiter action and in other systems the use of compression with complementary expansion may prove desirable.

It is highly desirable in microwave systems .to obtain such compression and expansion characteristics without recourse .to complicated circuit arrangementsinvolve theuse of one or more vacuum tubes.

Accordingly, it is an objectof thepresent invention to provide microwave circuits having non-linear transmission characteristics.suchthat they may be used to provide expander .orcompressor action or a .combinationof the two. It is a further objectof the-inventionto provide circuits for such applications which do not involve vacuum tubes and which are easily constructed and adjusted.

In accordance withthese objects the invention in one aspect relates to apparatus for use in modifying the amplitude range of an applied wave in accordancewith a .non-linearexpander characteristic and comprises a wave-guide junction having at leasttwo. pairs of conjugate arms or branches extending therefrom. Input and output connections are provided tor the respective branches of one of said'pairs and' a 'nonlinear impedance element is mountedin at least one of the branches of another" of these pairs. To provide for an expansion characteristic the impedance of the non-linear impedance element is matched tothat of the wave guide inwhich it is mounted at low signal energy levels. Other characteristics may be obtained by matching the non-linear impedance at someptl'ier amplitude level.

In another aspect, the invention comprises a structure of the type just described, modified however by the provision of identical non-linear impedance elements matched at the same level for each of the arms of a conjugate pair of arms not included in the input and output circuits, and the provision of phase shifting means in one of the impedance-terminated arms or branches to provide greater output for a given input than can be obtained with a single impedance element.

In still another aspect, the invention comprises expansion and compression circuits for'use at microwave frequenciessuch that the expansion or compressioncharacteristics may be varied in accordance with either signal amplitude or some external influence to provide a form of automatic gain control action.

The above and other features of the invention will be described in detail in the following specification taken in connection with the drawings in which: c

Fig. 1 is a block schematic diagram of a hybrid network having appropriate circuits in accordance with the invention;

Fig. 2 is a block schematic diagram of a modification of the circuit of Fig. l to permit the use of two impedance elements for the purpose of obtaining more efficient operation;

Figs. 3 and 4 are perspective drawings showing two alternative wave-guide structures corresponding to the block diagram of Fig. 2;

Figs. 5 and 6 are graphs illustrating the type of characteristics obtainable with the structures of Figs. 3 and l upon variations of certain of the circuit parameters;

Fig. '7 is a block schematic diagram illustrating a modification of the circuit of Fig. 2 to permit a form of automaticgain control action;

Fig. 8 is a block schematic diagram illustrating the application of the conventional automatic gain control techniquestolthe circuit of Fig.2;

Fig. 9 is a graph showingthe automatic gain control characteristic obtainable from the circuit of Fig. 7; and

Fig. 10 is a sectional drawing illustrating the construction of a typical crystal mounting of a type which may be employed in applicants invention.

"In Fig. 1 there is shown a hybrid network it to which appropriate circuit connections are made in accordance with the invention to permit modification of the amplitude ran'ge of signals applied from a source '12. The hybrid network which is essentially a bridge-type circuit may, for example, constitute the familiar wave-guide hybrid junction, sometimes known as the magic T which is described, for example, in an article entitled Hybrid Circuits for Microwaves by W. A. Tyrrell, Proceedings of the I. R. E., November 1947 at page 129%. Hybrid rings also described in the above-mentioned article, or directional couplers such as are described for ex- 3 ample in an article entitled Directional Couplers by W. W. Mumford, Proceedings of the I. R. E., February 1947 at page 160 may also be employed.

In hybrid circuits of this type a wave or signal incident, for example, upon transmission line I4 which is connected to source I2 is divided between transmission lines It; and 18 in such a way that no energy from source [2 is transmitted directly to transmission line 20 which forms the fourth arm of the network. Transmission lines 14 and 28 and transmission lines It and [8 form conjugate pairs of arms of the hybrid network or junction and are characterized by the fact that no energy may be transmitted directly from one arm of such a pair to the other and that the connections to the respective arms of a pair may be interchanged without modifying the performance of the entire circuit.

In the application of the circuit of Fig. 1 for the modification of the amplitude range of an input signal from source I2, one of the pair of conjugate arms [5, I8 is provided with a termination 22 which matches the characteristic impedance of the arm, while the other is terminated in a non-linear impedance 24 which in general will match the characteristic impedance of the arm only at one amplitude level. The fourth arm 20 is connected to a desired output circuit 25.

Assuming for the mom nt that it is desired to obtain an expansion characteristic such that input signals or" large amplitudes are relatively increased by greater amounts than input signals of small amplitudes, the non-linear impedance is matched to arm is at very low amplitude levels. If an input signal is applied from source 12, the incident energy transmitted to the hybrid network through arm i4 is divided therein and applied respectivel to arms i5 and ill, the energy incident upon arm (5 is completely absorbed by the terminatin impedance 22, and thus none is reflected back to the hybrid network On the other hand, and depending upon the amplitude level of the energ incident upon arm 58, a larger or smaller amount of energy will be reflected from non-linear impedance 24 and will return to hybrid network iii. If, as assumed above, an expansion characteristic is desired and non-linear impedance 24 is matched to arm 33 a low amplitude levels, low amplitude signals will be substantially completely absorbed in the non-linear impedance which matches the characteristic impedance of the arm at this energy level while progressively larger input signals will encounter a progressively more poorly matched termination in the form of non-linear impedance 24 and wiil correspondingly be reflected to hybrid network 50.

The wave reflected from non-linear impedance 24 is again divided in hybrid network ill and a portion of the energy is returned through transmission line 4 to source 2. The remainder of the reflected wave is transmitted through transmission line to the output circuit 26.

Clearly, a compression characteristic may be obtained from a circuit of this type by changing the level at which non-linear impedance 2 3 is matched to arm E8 from a low amplitude level to a high amplitude level approximating the maximum possible amplitude level which may ever be applied to the circuit from the input arm. Furthermore, if non-linear impedance 24 is matched to arm I8 at some other amplitude level, combination characteristics providing expansion action over a portion of the range 4 of incident energies and compression over another portion of the range may be obtained.

Fig. 2 illustrates a modification of the circuit of Fig. l to increase the efficiency thereof. In this circuit a wave from the source it is applied through transmission line [4 to a hybrid network iii which may be identical to that of Fig. 1. The conjugate arm transmission lines It and i8 are terminated in identical non-linear impedances 25 and 25 and a phase shifting network 28 is inserted in one of the conjugate arms of the pair not associated with the input and output circuits. In Fig. 2, for example, phase shifter 28 is shown as being inserted in transmission line The fourth transmission line 28 is connected to an output circuit 28 in a manner identical to that shown in Fig. 1.

In the circuit of Fig. 2 energy incident upon transmission line [4 is divided in hybrid network in and appears in conjugate arms i6 and i8. Non-linear impedances 24 and 25 which are preferably identical in character are matched to the respective arms i5 and IE3 at a desired amplitude level. Thus, if a compressor characteristic is desired, these non-linear impedances are matched to the arms in which the are mounted at high energy levels as in the case of impedance 24 in the circuit of Fig. 1. In each arm, therefore, the energy incident upon a nonlinear impedance is more or less reflected, depending upon the amplitude level of such energy, and returns to hybrid junction Ifl. Since, however, it is characteristic of a hybrid network of the type herein contemplated that the energy incident upon arms I6 and IS in response to the application of energy in arm i4 is in phase opposition, the reflected energies will return to the hybrid junction or network degrees out of phase with the result that with the identical terminations complete cancellation occurs within the network and no energy is available for application to output arm 28. It is for the purpose of obtaining a useful output that phase shifting device 28 is inserted in one of the pair of conjugate arms in which the non-linear im pedances are located. Phase shifting device 28 is adjusted to produce a phase difference in both the incident and reflected waves of 99 degrees. As a result, the two waves reflected from the nonlinear impedances return to hybrid network ill in phase With each other. Under these con ditions the two waves add in arm 29. On the other hand, the two waves add in phase opposition in arm I4 and are thus completely canceled. This is advantageous in that the reflected waves have no effect on the impedance which is presented to the incoming wave irom source I 2. The waves in branch 2 add on a voltage basis so that the total amount of energy transmitted into the output circuit is four times that which would be transmitted thereto in the circuit of Fig. 1 and is equal to the total power reflected from the two non-linear impedances.

Fig, 3 illustrates a wave-guide structure corresponding to the block diagram of Fig. 2 which may be employed to obtain either compression or expansion characteristics or a combination of the two types of characteristics. In this case the hybrid junction comprises a wave-guide hybrid T 36 which is of the well-known type referred to above. This junction may be considered to comprise three mutually perpendicular wave guides 38, 40 and 42-44 with the two sections 42 and 44 extending on opposite sides of the junction point and forming four arms 38, 40, 42 and 44. Of these arms 38 and 48 form one conjugate pair and arms 42 and 44 form a second conjugate pair. If, as indicated in Fig. 3, arm 38 is taken as the input arm no energy can emerge from output arm 48 by direct transmission from arm 38, this effect being common to all such hybrid structures. Arms 42 and 44 are terminated in wave-guide structures 46 and 48 which are conveniently identical and which provide means for terminating the ends of the respective arms in non-linear impedances. In one simple terminating structure of this type the non-linear impedance may as shown in Fig. comprise silicon or germanium crystal rectifiers I52 which are mounted directly within the wave-guide structure as by screws I54, one terminal of the crystal being connected directly to one wall of the structure and the crystal extending transversely of the structure in the direction of the electric vector. The other terminal of the crystal is coupled to the opposite wall of the structure through a small capacitor comprising washer I56, the adjacent wall of the wave-guide I58 and dielectric washer l68 to permit a connection to be brought out to an external circuit. In another arrangement such as is indicated in Fig. 3, probe-type crystal mounts 58 and 52 may be provided. These mounts may be of the type employed in crystal detectors for microwave circuits wherein the crystal is mounted in a short section of coaxial line supported exteriorly of the wave guide with the inner conductor of the line extending transversely of the wave guide to act as a probe in such a way that the entire structure forms a wave guide to coaxial line transducer.

An auxiliary section of wave guide 54 is connected in arm 44 between the hybrid junction 38 and the termination 48. Wave-guide section 54 is made of electrical length sufficient to provide a ISO-degree phase displacement between the wave incident upon arm 44 from the junction and that reflected back to the junction from the termination.

The terminating structures 48 and 48 and the non-linear impedances which may, for example, comprise crystals, varistors, thermistors or similar non-linear elements, are made identical. Thus, and as pointed out in connection with Fig.

energy incident upon arm 38 is divided substantially equally between arms 42 and 44 and. is transmitted therethrough to the non-linear impedances contained in terminations 46 and 48. Depending upon the amplitude level at which the non-linear impedances are matched to the respective arms and upon the incident wave energy, greater or smaller proportions of the incident energy are reflected back to the hybrid junction and appear in output arm 48. The reflected energies add voltage-wise by virture of the interposition of wave-guide section 54 which provides the requisite phase shift.

Fig. 4.- illustrates another form of wave-guide structure which may be employed in accordance with the invention. In this case a well-known form of directional coupler is employed. The directional coupler 56 comprises a section of wave guide 5?; which is coupled through orifices 68 to another section of wave guide 62 which is connected through QO-degree elbows to wave-guide arms ill and 66. This structure may b considered a four-arm hybrid junction, the arms comprising the two extensions 68 and 18 of wave guide 58 and arms 64 and 68. The conjugate pairs of arms in this instance comprise arms 66- and 88 and arms 64 and 18. As illustrated in Fig. 4, arms 66 and 68 have been chosen respectively as the output and input arms, while terminating structures 12 and 14 containing nonlinear impedance elements are applied to arms 64 and 18, respectively. The terminating structures at 12 and 14 may conveniently be identical to those employed in the structure of Fig. 3. An additional section of wave guide 16 is inserted between arm 78 and termination 14 and is made of electrical length sufficient to cause the inphase addition of the energies reflected from terminations T2 and 14 in output arm 66 as will be described hereinafter.

In this type of structure a portion of the energy incident in arm 88 is transmitted through orifices 88 into arms 84 and thus to the non-linear impedance at "2, the directional effect of thecoupling orifices 68 being such as to prevent the energy being transmitted to arm 66. Depending upon the level at which impedances 12 and 14 are matched to their respective arms more or less of the energy incident thereupon is reflected- A large portion of the reflected energy is transmitted through arm 66 to the output circuit while a smaller portion is transmitted direction-- ally through orifices 68 back through arm 68 to the source of energy. The greater portion of the energy incident in wave guide 68 is transmitted through arm l8 and phase delay section 16 to the non-linear impedance 14. The energy refiected from this non-linear impedance divides in orifices 68 so that a fraction of the reflected energy is transmitted through arm 66 to the output, while the major portion returns through arm 68 to the source. Th phase delay-introduced by wave-guide section 18 is made sufficient to provide for voltage addition of the energies reflected into arm 66 and to compensate for the difference in length of arms 64 and 18.

Inasmuch as a considerable portion of the incident energy is reflected back to the source in the system of Fig. 4 its performance is not as efficient as that of the hybrid T structure employed in the arrangement of Fig. 3. Further, because of the unequal transmission through the orifices 68 the waves reflected toward the source cannot be made to cancel with the result that the source will work essentially into the nonlinear impedance characteristic of non-linear impedance [4.

In the description of the structures of Figs. 3 and 4 it has been assumed that the non-linear impedances mounted in the terminating structures have been made identical and have been matched to the respective wave-guide arms in which they are mounted at a chosen energy level. Silicon crystal rectifiers for example have a nonlinear voltage versus current characteristic. If a sinusoidal wave is applied across the crystal the effective impedance thereof depends upon what part and how much of the voltage versus current characteristic is covered by the wave. When the level of the incident wave is changed, the part of the crystal characteristic which is effective to determine the impedance presented to the incident wave is also changed. Since the impedance so presented to an incident wave necessarily therefore has different values for incident waves of diiierent energy levels, the crystals employed in the terminations of the wave-guide structures of Figs. '3 and 4 canbe matched to the respective wave-guide arms at only oneenergy level and will in general present a mismatch to the respective wave guides at all other energy levels. If the crystal impedance is matched to that of the wave guide in which it is mounted for very small amplitudes of incident energy substantially Zero output will be obtained from the output arm for low level energy since substantially all such energy incident upon the crystals will be absorbed thereby. As the level of the incident energy is increased, however, the effective impedance of the crystals will change with the result that the amount of energy refiected from the crystal terminations Will increase and an output will appear in the output arm. Thenceforth, the amount of reflected energy increases relatively more rapidly than the increase in level of the incident energy and the crystals become progressively more poorly matched to the wave guides in which they are mounted and an input-output characteristic of the type shown by curve E8 of Fig. will be obtained. This characteristic will be recognized as a typical expander characteristic.

If, on the other hand, the crystals are matched to the respective wave guides at a high level of incident energy, the reflected wave amplitude is roughly proportional to the incident wave amplitude at low levels since the mismatch will then be substantially complete. As the amplitude of incident energy increases, however, the impedance of the crystals will begin to change and the ratio of incident energy to reflected energy will become smaller. Ultimately, further increase in incident energy will produce no further increase in reflected energy since maximum absorption thus occurs. As a result the reflected wave amplitude remains substantially constant upon increase of the incident energy beyond a certain level. This type of characteristic is shown by curve 79 of Fig. 5 which may be recognized as a typical limiter characteristic.

In the adjustment of expander or compressor circuits in accordance with the invention it is necessary only to apply an input wave of appropriate energy level and adjust the position and/ or other characteristics of the crystal terminations to obtain minimum output. Thus to obtain an expander characteristic, a test signal until the output meter indicates a minimum.

Similarly, these devices may be adjusted to operate as compressors or limiters by applying a test signal of very high energy level to the input arm and again adjusting the crystal terminations for minimum output.

Fig. 6 is a graph illustrating various characteristics obtained from a circuit of the type shown in Fig. 3. These characteristics were obtained by applying external bias voltages to the crystal rectifiers thereby to assist in fixing the impedance thereof. Curve 8B for example illustrates the characteristic obtained with zero effective bias when the non-linear impedances have been matched to the respective wave guides in which they are mounted at low energy levels. The remaining curves 82, 84, 86, 88 and 96 were obtained by changing the effective bias upon the crystal rectifiers without, however, changing the adjustments whereby the rectifiers were matched to the arms in which they were mounted. This effectively results in changing the energy level at which the non-linear impedances are matched to their wave guides and these curves therefore also illustrate the type of combination characteristic which may be obtained by matching the non-linear impedances to their respective arms at energy levels intermediate the limits of the range of incident energies applied to the system. Thus in curve 84 and progressively more so in curves 86, 88 and 90 only a portion of the characteristic corre sponds to the conventional expander characteristic. In each case the right-hand portion of the curve, shown solid in Fig. 6, is a conventional expansion characteristic which, however, is shown as reaching a well-defined minimum at which a marked discontinuity occurs. The lefthand portions of the curves, shown dashed in Fig. 6, correspond to compression characteristics and it is thus recognized that by matching the non-linear impedance or impedances to the wave-guide arms on which it or they are mounted at chosen levels, various combination characteristics may be obtained. For example, a characteristic exhibiting expansion at low levels and compression at higher levels may also be obtained. The expansion and compression char acteristics of Fig. 6 differ in appearance from those of Fig. 5 because the former characteristics are plotted on linear coordinates while the latter are plotted on a decibel scale.

Fig. 7 illustrates a modification of the circuit of Fig. 2 to provide a form of automatic gain control action. Such action is obtained by taking advantage of the fact that the expander or compressor characteristic may be varied by changing the bias effectively applied to the nonlinear impedance as illustrated by the curves in Fig. 6. This may be accomplished by taking advantage of the self-rectification which may be caused to take place in the crystal rectifiers by providing a suitable external circuit therefor. Thus in Fig. 7 the wave-guide hybrid network I96 is provided with input and output arms Hi2 and IE4 and with paired arms I and lei-5. Arms I06 and I08 are shown as terminated by crystal rectifiers H0 and H2 respectively and a quarter-wave phase delay section II is inserted in arm I08. Crystal rectifiers H6 and I I2 are connected across arms I at and I08 re spectively through capacitors H6 and H8 and in each case the junction of the rectifier and the capacitor is connected through a resistor I20 to the positive terminal of a battery $22, the negative terminal of which is connected to ground. Energy incident upon rectifiers I it) and H2 is rectified therein and a corresponding voltage drop appears across resistor IZU. This provides a bias voltage which appears effectively across the crystals and thus determines the impedance offered thereby to the incident energy. his impedance will therefore vary with the level of incident energy and a form of automatic gain control action may thus be obtained. Curves I24 and I25 of Fig. 9 show variations in peak expander gain with peak input level for different values of resistor I20. An additional fixed bias may also be applied to the rectifiers H0 and H2 by the use of a battery I22. This has the efiect of modifying the automatic gain control characteristic obtained by the self-rectification action to give a combined characteristic such as is shown by curve I23 in Fig. 9.

I The expanders and compressors of the invention also lend themselves to the application of conventional automatic gain control techniques. Thus in Fig. 8 the circuit comprising hybrid network I 30 with input and output arms I32 and I34 and non-linear impedance arms I36 and I38 containing non-linear impedances I40 and I42 respectively, may have applied thereto a more or less conventional automatic gain control circuit. Accordingly, a portion of the output appearing on arm I34 is applied through T junction I44 to a crystal rectifier I46 the rectified output of which is a direct-current voltage proportional to the amplitude of the energy applied to output I48. This direct-current voltage is amplified in an amplifier I50 the output of which is applied to non-linear impedances I40 and I42 to vary the effective bias thereon and thus to provide automatic gain control action.

What is claimed is:

1. Apparatus for modifying the amplitude range of applied waves comprising a wave-guide directional coupler having a main guide and a branch guide directionally coupled thereto, each of said guides providing a pair of arms extending from the coupling, an input connection to an arm of said main guide, a non-linear impedance in the branch guide arm directionally coupled to said input arm, an output connection for the remaining arm of said branch guide, a non-linear impedance identical to the first-mentioned non-linear impedance mounted in the other arm of said main guide, said non-linear impedances being matched to said arms at the same amplitude level, and means associated with one of said arms for phasing the energy reflected from the non-linear impedance mounted therein to appear in the output arm in phase with that reflected from the other non-linear impedance.

2. Apparatus for modifying the amplitude range of applied waves comprising a wave-guide directional coupler having a main guide and a branch guide directionally coupled thereto, each of said guides providing a pair of arms extending from the coupling, an input connection to an arm of said main guide, a crystal rectifier in the branch guide arm directionally coupled to said input arm, an output connection for the remaining arm of said branch guide, a crystal rectifier identical to the first-mentioned crystal rectifier mounted in the other arm of said main guide, said crystal rectifiers being matched to said arms at the same amplitude level only and a wave-guide section interposed between said coupling and the crystal rectifier in one of said arms, said waveguide section having an electrical length such that the round trip delay of the energy transmitted to and reflected from the crystal rectifier in that arm will be sufiicient to cause addition in phase of the energy reflected therefrom and that reflected from the crystal rectifier in the other of said arms.

3. Apparatus for modifying the amplitude range of applied Waves comprising a wave-guide junction having at least two pairs of conjugate arms extending therefrom, input and output connections for the respective arms of one of said pairs, identical non-linear impedances mounted respectively in the arms of the other of said pairs, one of said non-linear impedances being mounted one-quarter wavelength further from said junction than the other, and said non-linear impedances being matched to the respective arms in which they are mounted at a chosen signal amplitude level, and means including a circuit connected to said crystals to permit self-rectification of the energy incident thereupon thereby to change the impedances of said crystals with amplitude level.

4. Apparatus for modifying the amplitude range of applied waves comprising a wave-guide junction having at least two pairs of conjugate arms extending therefrom, input and output connections for the respective arms of one of said pairs, identical element so mounted respectively in the arms of the other of said pairs as to interact With the energy therein and having impedances matching those of the last-mentioned arms at a chosen level of said energy but varying with variations in the level of said energy, one of said elements being mounted one-quarter wavelength further from said junction than the other, means for developing a direct-current bias voltage across said elements varying with the amplitude level of the energy incident thereupon, said means comprising an external circuit including said elements and a load impedance for said elements, and means for applying a fixed bias voltage to said elements.

5. Apparatus for modifying the amplitude range of applied waves comprising a wave-guide junction having at least two pairs of conjugate arms extending therefrom, input and output connections for the respective arms of one of said pairs, identical elements so mounted respectively in the arms of the other of said pairs as to interact with the energy therein and having impedances matching those of the last-mentioned arms at a chosen level of said energy but varying with variations in the level of said energy, one of said elements being mounted one-quarter wavelength further from said junction than the other, and means for developing a direct-current voltage varying with the energy appearing at said output connection and means for applying said varying voltage to said elements to vary the impedance thereof.

6. Apparatus for modifying the amplitude range of applied waves, comprising a wave guide junction having at least two pairs of conjugate arms extending therefrom, input and output connections for the respective arms of one of said pairs, identical elements so mounted respectively in the arms of the other of said pairs as to interact with the energy therein and having impedances which match those of the last-mentioned arms at a chosen level of said energy but vary with variations in the level of said energy, one of said elements being mounted one-quarter wavelength further from said junction than the other, and means for developing an automatic gain control voltage for application to said elements comprising means for rectifying a portion of the energy appearing in the output connection of said apparatus, and an amplifier connected between said rectifying means and said elements.

CASSIUS C. CUTLER.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,475,474 Bruck July 5, 1949 2,484,256 Vaughan Oct. 11, 1949 

